1. Field of the Invention
This invention relates to an image forming apparatus such as a copying apparatus, a printer or a facsimile apparatus, and particularly to an image forming apparatus capable of forming two-color images.
2. Related Background Art
A method of forming two-color images according to the prior art will hereinafter be described.
The surface of an electrophotographic photosensitive drum (photosensitive medium) is uniformly charged by a charger, whereafter first exposure is effected to form a first latent image, which is then developed by a first developing device by the use of a first toner, and then the surface of the photosensitive drum is again uniformly charged by a re-charger, whereafter second exposure is effected to form a second latent image, which is then developed by a second developing device by the use of a second toner differing in color from the first toner, whereby a two-color image by the toners of two colors is formed on the photosensitive drum. This is a so-called two-color multiplex developing method.
Such a two-color multiplex developing method has suffered from the problem that the first toner, adhering to the first latent image on the photosensitive drum in the development, mixes into the second developing device with an increase in the number of image formation sheets and the turbidity of the colors occurs on the two-color image obtained.
This problem is remarkable particularly when the two-component magnetic brush developing method is used in the second development. This is because the first toner, adhering to the first latent image on the photosensitive drum, is mechanically stripped off by its contact-with the ears of the magnetic brush of the two-component developer.
Accordingly, to prevent the mixing of the first toner into the second developing device, it is more advantageous to use in the second development a non-contact developing method in which the first toner is not stripped off by the contact of the second toner. Further, by applying to the second developing device an AC bias (AC electric field) or an alternate bias (alternate electric field) comprising a DC voltage superposed on an AC voltage as a developing bias, the qualities of image such as density and harmony are improved.
As the technique of preventing the mixing and color-mixing of the toner in the developing device on the rear stage, it has been proposed to make the particle diameter of the toner in the developing device on the rear stage larger (Japanese Laid-Open Patent Application No. 58-82263), to make the amount of charge of the toner great (Japanese Laid-Open Patent Application No. 58-137846) or to make the mass, the average particle diameter and the specific gravity of the toner great (Japanese Laid-Open Patent Application Nos. 61-7852, 63-294579 and 63-294580).
However, the mixing of the first toner is not entirely eliminated by even such measures. That is, when the first toner of a negative polarity, adhering to the first latent image of the negative polarity on the photosensitive drum by the first development, has arrived at a second developing area on which the developing sleeve of the second developing device is disposed with the rotation of the photosensitive drum, the first toner and the second toner exhibit the same behavior because they are of the same polarity, and in some cases, the first toner on the photosensitive drum also reciprocally moves between the photosensitive drum and the developing sleeve of the developing device and is stripped off the photosensitive drum and mixes into the second developing device.
The foregoing will hereinafter be explained by the use of the potential model of FIG. 6 of the accompanying drawings. FIG. 6 shows the change of the charging potential of the surface of the photosensitive drum when the photosensitive drum has been uniformly charged to the negative by corona charging, exposed, and developed. In FIG. 6, VD1 indicates the potential of an unexposed portion, and VL1 indicates the potential of the first latent image, i.e., the potential of the exposed portion of the first image. VT1+L1 indicates the potential of the toner layer on the photosensitive drum, and VDC1 indicates the DC voltage of the first developing bias (AC voltage+DC voltage) when the exposed portion of the first image of VL1 is reversely developed. Also, VDC2 indicates the DC voltage of the second developing bias (AC voltage+DC voltage) when the exposed portion of the second image (the second latent image) is reversely developed.
The first image portion obtained by the exposed portion of the first image on the photosensitive drum being subjected to the first development has its potential VL1+T1 raised to a potential VL1+T1' by re-charging. This potential after the re-charging is set to a level higher than the DC bias DC2 of the second development in order to prevent the second toner from adhering to the toner image of the first image portion on the photosensitive drum during the second development to thereby cause fogging. Thus, the converse bias Vb of the DC part for stripping in the figure is applied and it may happen that the first toner is stripped off. Accordingly, 100% prevention of mixing is very difficult and during long-period use, with an increase in the number of image formation sheets, the first toner mixes into the second developing device via the developing sleeve thereof, and accumulates therein to thereby cause color mixing.